Embodiments of the present invention disclosed herein generally relate to a method and apparatus for detecting rotating stall in a compressor, in particular in a centrifugal compressor.
Rotating stall, also known as rotational stall, is a local disruption of airflow within a compressor which continues to provide compressed fluid but with reduced effectiveness.
Rotating stall arises when a small proportion of aerofoils experience aerofoil stall, disrupting the local airflow without destabilizing the compressor. The stalled aerofoils create pockets of relatively stagnant fluid (referred to as stall cells) which, rather than moving in the flow direction, rotate around the circumference of the compressor. The stall cells rotate with the rotor blades, but at a lower speed, affecting subsequent aerofoils around the rotor as each encounters the stall cell.
A rotating stall may be momentary, resulting from an external disturbance, or may be steady as the compressor finds a working equilibrium between stalled and unstalled areas. Local stalls substantially reduce the efficiency of the compressor and increase the structural loads on the aerofoils encountering stall cells in the region affected.
In many cases, however, the compressor aerofoils are critically loaded without capacity to absorb the disturbance to normal airflow such that the original stall cells affect neighboring regions and the stalled region rapidly grows to become a complete compressor stall which is commonly known as surge. If surge continues and no action is taken to stop it, the rotor blades will be severely damaged and, eventually, the whole compressor will be damaged.
Therefore, it is important to try to avoid surge in a compressor.
U.S. Pat. No. 6,092,029 discloses a method and an apparatus for diagnosing rotating stall of a rotating machinery by monitoring dynamic shaft precession of the machine and comparing this precession with a standard one and altering the precession as the machine approaches a destabilizing condition when indicated by the comparison step. Axial vibration monitoring means is also provided for monitoring and comparing a dynamic axial vibration of the machine with that of a standard one and altering the axial vibration as the machine approaches a destabilizing condition when indicated by the comparison step. Furthermore, the complex dynamic stiffness of the machine is measured and the direct dynamic stiffness and the quadrature dynamic stiffness are computed for use as a destabilizing warning.
U.S. Pat. No. 6,532,433 discloses a method and an apparatus for continuous prediction, monitoring and control of compressor health via detection of precursors to rotating stall and surge; at least one sensor is operatively coupled to the compressor for monitoring at least one compressor parameter. According to the embodiments, a plurality of sensors are disposed about the casing of the compressor for measuring dynamic compressor parameters such as, for example, pressure, velocity of gasses flowing through the compressor, force, vibrations exerted on the compressor casing. A system is connected to the sensor for computing stall precursors. According to an embodiment, compressor data are measured as a function of time, FFT is performed on the measured data and changes in magnitudes at specific frequencies are identified and compared with baseline compressor values.
US2004/0037693 discloses a system and method for detecting rotating stall in a centrifugal compressor, particularly in the diffuser region of a centrifugal compressor. The process begins with the detection or sensing of acoustic energy associated with the onset of rotating stall. A pressure transducer is placed in the gas flow path downstream of the impeller, preferably in the compressor discharge passage or the diffuser, to measure the sound or acoustic pressure phenomenon. Next, the signal from the pressure transducer is processed either using analog or digital techniques to determine the presence of rotating stall. Rotating stall is detected by comparing the detected energy amount, which detected energy amount is based on the measured acoustic pressure, with a predetermined threshold amount corresponding to the presence of rotating stall.
US2010/0296914 discloses a stall and surge detection system and method for a compressor. The system comprises a vibration monitor that monitors radial vibrations, axial vibrations and axial displacement. According to a first embodiment, radial vibrations in one fixed and predetermined frequency bandwidth based on the minimum operating rotating speed of the rotor of the compressor, specifically from 2.5 Hz to 45 Hz, are monitored for detecting incipient surge, i.e. rotating stall. According to a second embodiment, using a tracking filter, tracked to the rotational frequency of the rotor of the compressor, radial vibrations in the range of frequencies from e.g. 5% of the rotational frequency to e.g. 90% of the rotational frequency are monitored for detecting incipient surge, i.e. rotating stall.
WO2009/055878 discloses a method to avoid instable surge conditions with centrifugal compressors. The method provides to measure and/or calculate forces on the bearings of the rotor of the compressor, and to detect timely exceptional imbalance of radial forces on the bearings which occurs before the centrifugal compressor ends up in an unstable condition. According to one embodiment, the component of the radial forces which is synchronous with the rotational frequency of the rotor is eliminated.
Therefore, there are solutions in the prior art that detect one or more indicators of an incipient surge in a compressor; some of these known solutions monitor the axial vibration of the compressor.
There is still a need for a solution to the problem of detecting incipient surge that is accurate, simple and flexible.